1. Field of the Invention
The present invention relates to a linear motor, and more particularly to an improved linear motor which makes it possible to carry out a speed control without an additional current control system, by appropriately shifting a clearance between a coil unit and a moveable unit, and a magnetic flux area with regard to a relation of the coil unit and the moveable unit.
2. Description of the Background Art
In general, a linear motor is formed of a coil unit and a moveable unit, and includes a link, a chain and other instruments. The linear motor serves to shift a rotational energy to a linear energy.
Such a linear motor becomes further simplified and less heavy, and significantly reduces energy and space required therein, thereby being applicable to a variety of appliances such as conveyer and elevator.
As shown in FIGS. 1A and 1B, in a conventional linear motor 100, a coil unit 9 is provided with a core laminations 12 and a plurality of protrusions 12a. A cylindrical coil unit 11 is formed between the protrusions 12a.
Below the coil unit 9 there is provided a moveable unit 10 which includes a magnetic plate 10a having a plurality of protrusions 10a' thereon and a metallic support plate 10b formed beneath the magnetic plate 10a.
Here, a clearance .delta. is maintained between the protrusions 12a formed on the lower surface of the core laminations 12 in the coil unit 9 and the protrusions 10a' formed on the upper surface of the moveable unit 10.
In order to constantly maintain the clearance .delta., a roller 13 is provided between each end of the support plate 10b of the moveable unit 10 and the core laminations 12 of the coil unit 9.
The thusly constituted conventional linear motor will be described with reference to an elevator system applied thereto.
As shown in FIG. 2, in an elevator body 1 of an elevator system employing the conventional linear motor, respective lower end portions of a left and right side power transfer plates 4, 5 are fixedly connected to corresponding upper portions of left and right doors 2, 3 which serve to open/close the elevator system 1.
A rotation rope 8 that circulates on a pair of auxiliary pulleys 6, 7 formed at each side portion of the elevator body 1 is fixedly connected to an upper portion of each of the power transfer plates 4, 5 so as to horizontally move therealong.
Here, the linear motor 100, the coil unit 9 of which is fixed to a portion of the elevator body 1, supplies a driving power to the left and right power transfer plates 4, 5. The moveable unit 10 of the linear motor 100 is fixed to either of the left and right power transfer plates 4, 5, and faces against the coil unit 9. Also, a predetermined clearance .delta. is maintained between the plurality of core laminations protrusions 12a of the coil unit 9 and the moveable unit 10.
A current control system 20 is installed at a side portion of the elevator body 1 so as to control a power supply for controlling the doors 2, 3 in connection with the coil unit 9. A rotary encoder 21 connected to the power controller 20 is attached on a shaft of the auxiliary pulley 7 so as to detect a close/open speed of the elevator doors 2, 3.
As shown in FIG. 3, the current control system 20 includes: a power source 22; a converter 23 connected by a diode bridge to the power source 22 and converting the supplied current to a direct current; an inverter 24 composed of a switching device such as a transistor, converting the direct current converted in the converter 23 to a three-phase alternating current having a predetermined frequency, and releasing the converted alternating current to respective wires u, v, w of the core laminations 12 in the linear motor 100; a switching command generator 25 generating a switching command with regard to the left and right doors 2, 3 of the elevator body 1; a detector 26 detecting a location of the moveable unit 10 of the linear motor 100; a magnetic processing unit 27 receiving a signal from the detector 27 and determining the location of the moveable unit 10 of the linear motor 100; a command distributor 28 commanding a magnetization of the respective wires u, v, w of the linear motor 100 in accordance with a signal outputted from the magnetic processing unit 27; an arithmetic unit 29 receiving the respective signals from the switching command generator 25 and the command distributor 28, and obtaining a call order and time in a transistor group of the inverter 24; and a rotary encoder 21 detecting the location of the moveable unit 10 of the linear motor 100.
With reference to FIGS. 1A through 3, the operation steps of the conventional linear motor applied to the elevator body 1 will now be described.
First, when there occurs a door open command after the arrival of the elevator body 1 to a target floor, the current control system 20 supplies an electrical power to the core laminations 12 of the coil unit 9 so that the moveable unit 10 can move from left to right.
Accordingly, the moveable unit 10 moves the to right, and the right power transfer plate 5 moves toward the right side, so that the right door 3 connected to the right power transfer plate 5 moves toward the right side. In accordance with the moveable of the right side door 3, the rotation rope 8 is rotated and accordingly the left power transfer plate 4 moves toward the left side, whereby the left side door 2 is opened.
At this time, the rotary encoder 21 is rotated anticlockwise, and the moveable momentum of the left and right doors 2, 3 is detected, and the signal detected in accordance with the rotary encoder 21 is transferred to the current control system 20, thereby controlling the speed and earthing of the left and right doors 2, 3.
When the left and right doors 2, 3 are completely opened, the power supply from the current control system 20 is interrupted, and the left and right doors 2, 3 are earthed.
The closing operation of the left and right doors 2, 3 is carried out in a reverse order.
However, because the conventional linear motor employs a variable control of the current so as to convert a torque for controlling the door close/open speed of the elevator body, there is inevitably required an additional speed control apparatus, thereby significantly increasing production cost and deteriorating productivity.